In many applications, such as for instance mobile terminals and handheld devices, it is well known that the size of the device restricts the size of the antenna and its ground plane, which has a major effect on the overall antenna and terminal performance. In general terms, the bandwidth and efficiency of the antenna and terminal device are affected by the overall size, geometry, and dimensions of the antenna and the ground plane. A report on the influence of the ground plane size in the bandwidth of terminal antennas can be found in the publication “Investigation on Integrated Antennas for GSM Mobile Phones”, by D. Manteuffel, A. Bahr, I. Wolff, Millennium Conference on Antennas & Propagation, ESA, AP2000, Davos, Switzerland, April 2000. In the prior art, most of the effort in the design of antennas including ground planes (for instance microstrip, planar inverted-F or monopole antennas) has been oriented to the design of the radiating element (that is, the microstrip patch, the PIFA element, or the monopole arm for the examples described above), yet providing a ground plane with a size and geometry that were mainly dictated by the size or aesthetics criteria according to every particular application.
Volume and size are typically an important aspect of a portable radio device, such as for instance a hand-held telephone (cellular phone, mobile/handset phones, smart phone, e-mail phone) or a wireless personal digital agenda (PDA) or computer. From the consumer's perspective the overall volume, mechanical design, ergonomics and aesthetics of the phone are critical. For instance, there has been an increasing trend in removing external antennas from handsets and substituting them by internal antennas that conveniently fit inside the phone. This solves the problem of removing a protruding part of the phone. External antennas feature several drawbacks: they can break accidentally under mechanical stress or shock and they make the phone more inconvenient and uncomfortable to carry inside a pocket and to extract it outside for operation. For the same reason, there is an increased trend in making slimmer, thinner phones that can better fit inside for instance a shirt or jacket pocket or a bag or case.
The desire to make smaller, thinner phones may conflict with the trend of adding more features to the phone. On one side, phones are increasingly adding components and features such as large color screens, digital cameras, digital music players (MP3, WAV), digital and analogue radio and multimedia broadcast receivers (FM/AM, DAB, SDARS, DMB), web browsers, QWERTY keyboards, satellite receivers and geolocalization systems (GPS, Galileo, Sirius, SDARS) and come with a wider range of form factors (candy bar phones, clamshell phones, flip-phones, slider phones, . . . ). Also, from the communication perspective, new cellular and wireless services are being added, which in some cases means that multiband capabilities are required (to feature several standards such as for instance CDMA, GSM850, GSM900, GSM1800, PCS1900, UMTS, WCDMA, Korean PCS) or that other connectivity components are to be included (for instance for Bluetooth, IEEE802.11 and IEEE802.16 services, WiFi, WiMax, ZigBee, Ultra WideBand). These trends put an increasing pressure on the antenna features, which need to feature a small footprint, a thin mechanical profile, yet performing efficiently at one or more frequency bands.
There is a well know trade-off between size of the antenna and performance. The fundamental limits on small antennas where theoretically established by H. Wheeler and L. J. Chu in the middle 1940's. They basically stated that a small antenna has a high quality factor (Q) because of the large reactive energy stored in the antenna vicinity compared to the radiated power. Such a high quality factor yields a narrow bandwidth; in fact, the fundamental derived in such theory imposes a maximum bandwidth given a specific size of a small antenna. Related to this phenomenon, it is also known that a small antenna features a large input reactance (either capacitive or inductive) that usually has to be compensated with an external matching/loading circuit or structure. It also means that is difficult to pack a resonant antenna into a space which is small in terms of the wavelength at resonance. Other characteristics of a small antenna are its small radiating resistance and its low efficiency.
Searching for structures that can efficiently radiate from a small space has an enormous commercial interest, especially in the environment of mobile communication devices (cellular telephony, cellular pagers, portable computers and data handlers, to name a few examples), where the size and weight of the portable equipments need to be small. According to R. C. Hansen (R. C. Hansen, “Fundamental Limitations on Antennas,” Proc. IEEE, vol. 69, no. 2, February 1981), the performance of a small antenna depends on its ability to efficiently use the small available space inside the imaginary radian sphere surrounding the antenna.
The internal antenna of a cell phone usually takes the form of a substantially planar conducting element placed at a distance over the PCB substrate that includes the electronic circuitry of the handset. In most of the cases, one of the conducting ground layers in the PCB cover a substantial part or even the whole area of the footprint underneath the antenna. The advantage of this is that such a ground layer shields the antenna from the backward side of the PCB, therefore allowing for additional space for other components (such as for instance earpiece, vibrator, RF connectors, LCD screen, speakers, chips, RF and electronic circuitry . . . ) therefore allowing for a substantial integration and compactness of the whole device. One of the drawbacks of this is that having the antenna on one side of the PCB and other components on the back side of such a PCB implies a minimum thickness for the whole handset device.
Usually, antennas with a substantially planar conducting element placed at some distance over a ground layer are known as microstrip or patch antennas. Usually such microstrip and patch antennas include at least a feeding contact and a short to ground contact, forming a so called Planar Inverted F Antenna (PIFA). It is well known that the performance of such antennas is limited, in terms of bandwidth, efficiency and related parameters (gain, VSWR and so on) by the spacing between said conducting element and the ground layer: the shorter the distance between both, the smaller the bandwidth and efficiency. For the typical 5-15% bandwidths of a cellular/mobile system (GSM, UMTS, PCS, WCDMA), the minimum distance is about 2% of the longest operating wavelength (typical 7-9 mm), which again introduces a significant limitation in the development of thin, slim phones with multiple-band or wide-band operation.